Abstract

Breast cancer is composed of heterogeneous cell populations with different biological properties. The capacity to form tumors resides in a small group of cells termed tumor initiating cells or cancer stem cells. Tumor initiating cells have been identified in a variety of cancers by sorting of subpopulations based on cell surface markers and transplantation into animal models. Tumor initiating cells have the important feature of self renewal, which is a property in common with stem cells. We examined established breast cancer lines for cells with tumor initiating properties. A dye efflux side population in MCF7 and T47D lines expressed markers of breast cancer stem cells. The side population represents a distinct morphologic and functional subpopulation within the human breast cancer cell lines MCF7 and T47D. The side population from human breast cancer cell lines was able to initiate tumors in vivo. The side population cells from human breast cancer cell lines were more resistant to ionizing radiation than the non-side population. We concluded that tumor initiating cells exist in established human breast cancer cell lines.